With a conventional electronic sphygmomanometer, an arm band (cuff) is wrapped around a measurement site, the pressure inside the cuff (cuff pressure) is increased to greater than the highest blood pressure, and a pulse produced by an artery is detected with a pressure sensor via the cuff in a subsequent process of gradually reducing the cuff pressure, with the highest blood pressure and the lowest blood pressure being decided utilizing the cuff pressure and the magnitude of the pulse (pulse wave amplitude) at that time (oscillometric method). In contrast, a sphygmomanometer employing a volume compensation method that is configured to continuously measure blood pressure every heartbeat in a noninvasive manner has been developed (Patent Literature 1).
The volume compensation method involves continuously detecting blood pressure values by compressing an artery with a cuff from outside the body, equalizing the compression pressure (cuff pressure) with arterial pressure, that is, blood pressure, by keeping the volume per unit length of the pulsating artery constant, and detecting the cuff pressure when this state is maintained. In the volume compensation method, the volume (control target value “V0”) of the artery in a state where the arterial pressure is in equilibrium with the cuff pressure exerted on the artery, that is, when the arterial wall is in an unloaded state, is detected in advance. The cuff pressure is controlled such that the artery volume which changes depending on the pulse per heartbeat matches the control target value V0 (servo control).
With conventional servo control, PID control for controlling the cuff pressure is used, using the difference between an arterial volume signal (DC component of volume pulse wave) and a target arterial volume signal as a feedback signal. With this control method, increasing the control gain until an arterial volume change signal (AC component of volume pulse wave) is less than or equal to −15 dB of the gain at the time of the maximum amplitude is said to enable a blood pressure measurement error of approximately 5% to be achieved (Non-Patent Literature 1).    Patent Literature 1: JP 54-50175A